From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [net-next PATCH 2/5] ethernet/intel: Use eth_skb_pad helper
Date: Tue, 25 Nov 2014 20:01:13 -0800
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To: David Miller <davem@davemloft.net>
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On Tue, 2014-11-25 at 22:19 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Tue, 25 Nov 2014 17:43:05 -0800
> 
> > I believe I finally have an idea why we had various + 15 in skb
> > allocations in TCP stack !
> 
> It was so that you could do one level of tunneling with "for
> free".  Or that is my recollection.
> 
> Those + 15 existed way before any of these padto() calls even
> existed.


Well, tunneling is added in front of the packet. Thats why we use
MAX_TCP_HEADER.

The +15 is in fact because TCP stack wanted to make sure the eventual
padding (needing tailroom, not headroom) was possible...

Note that ack packets never used the +15, but other packets did.